In connection with the recent widespread use of mobile phones or advances thereof, high-speed, large-capacity signal transmission has become an essential technology. Superconductors have significantly smaller surface resistance even in a high frequency region than typical electric conductors. Use of a superconductor can thus achieve a low-loss, high-Q resonator, and a resonator using a superconductor (superconductive device) is a promising device as a filter for a mobile communication base station.
When such a superconductive device is applied to a bandpass filter on the receive side, it is expected that the bandpass filter has low signal transmission loss and a sharp frequency cutoff characteristic. On the other hand, when such a superconductive device is applied to a bandpass filter on the transmit side, it is expected that the bandpass filter can remove signal distortion generated in a power amplifier disposed in the front end stage. However, in such a case, there is a problem of requiring a large amount of power for transmitting high frequency signals. That is, when a superconductive device is applied to a bandpass filter on the transmit side, achieving size reduction and a satisfactory power characteristic at the same time is an immediate challenge.
When a superconductive device is applied to a bandpass filter on the transmit side, there is a problem with transmission loss due to high RF power input. This problem of the transmit loss is, in other words, a problem of current density concentration. To eliminate this problem of current density concentration, it has been proposed that a linear pattern, such as a hairpin pattern and a microstrip pattern, be replaced with a disc (circular) shape superconductive resonator pattern. In a disc shape superconductive resonator pattern, the current density concentration at a linear edge can be reduced, as compared to a linear superconductive resonator pattern.
As shown in FIGS. 1A and 1B, there has also been proposed a dual-mode filter having a notch (cutout) in part of a disc superconductive resonator pattern, which induces resonant modes in two direction perpendicular to each other. (For example, refer to Sang Yeol Lee, Kwang Yong Kang, and Dal Ahn, IEEE Transactions on Applied Superconductivity, Vol. 5, No. 2, page 2567, June 1995.)
As shown in FIGS. 1A and 1B, in the conventional dual-mode resonator filter, a disc resonator pattern 102 formed of a superconductive film is provided on a dielectric substrate 101. FIG. 1B is a schematic cross-sectional view of the resonator shown in FIG. 1A taken along the line that connects input/output signal lines 103 to a notch 105. The notch 105 is formed in part of the disc resonator pattern 102 to induce dual modes, as best seen in FIG. 1A. In the conventional dual-mode resonator filter, the notch 105 is provided at a position apart from the extensions of the input/output signal lines (feeders) 103. The back side of the dielectric substrate 101 is entirely covered with a ground film 104, as shown in FIG. 1B.
FIG. 1C is a graph showing the frequency response characteristic of the superconductive resonator filter shown in FIGS. 1B and 1C. In the graph shown in FIG. 1C, the horizontal axis represents the frequency (GHz), and the vertical axis represents the magnitude (dB) indicative of the signal transmission characteristic.
As a result of the simulation, the reflective characteristic (S11) and the transmissive characteristic (S21) shown in FIG. 1C are obtained. Specifically, the sample has two resonant frequencies. 4.862 GHz (f1) and 5.051 GHz (f2), in the 5-GHz band.
In such a dual-mode resonator filter, degeneracy of the electric and magnetic modes perpendicular to each other is broken to separate resonant frequencies, so that two resonant frequencies f1 (on the low frequency side) and f2 (on the high frequency side) are generated, as shown in FIG. 1C. However, provision of the notch 105 causes the problem of current density concentration at the corners of the notch 105.
FIGS. 2A and 2B show the results of simulation of current density distribution in the conventional notch-type dual resonator. As shown in FIG. 2A, the current on the low frequency f1 side (especially compared to the High frequency f2 side concentrates at the corners of the notch 105 in the resonator plane, and hence exceeds the maximum current density in a typical disc resonator without the notch 105. In this example, a maximum current density of 702 A/m is induced on the f1 side, as shown in the left hand drawing in FIG. 2A.
On the other hand, as shown in FIG. 2B, since the back side (ground plane) of the dielectric substrate 101 is entirely covered with a superconductive film, there is substantially no local concentration of current density. It is however noted that the current density slightly rises around the center of the substrate along the current directions in the two resonant modes. The current directions in the two resonant modes refer to a first direction toward the notch 105 and a second direction perpendicular to the first direction. The resonant frequencies f1 and f2 are out of phase by 45 degrees with respect to each other at the maximum current density.
As described above, in the conventional dual-mode resonator filter, the current density concentrates at the corners or the edge of the notch 105 in the resonator pattern 102. As a result, in a bandpass filter and an antenna using a superconductive resonator, the withstanding power, which is the allowable power value (allowable power), is reduced, or the signal distortion increases in a disadvantageous manner.
Accordingly, the conventional technology cannot provide a superconductive device with high power handling capability and reduced concentration of the current density (signal distortion).